This study has five specific objectives: 1. To compare the effectiveness of behavior therapy + placebo to behavior therapy + sibutramine in reducing weight during 6 initial months of treatment (Phase 1). 2. To compare the above two conditions in changes in: 1) waist circumference; 2) blood pressure; 3) serum lipids; 4) plasma glucose; 5) psychological status; and 6) eating behavior. 3. During Phase 2, subjects treated by placebo during Phase 1 will be placed on sibutramine from months 6-12. The aim will be to assess the efficacy in inducing further weight loss from 6-12 months by the addition of sibutramine to this group initially treated by behavior therapy + placebo during Phase1. 4. Those subjects initially on sibutramine (during Phase 1) will continue on sibutramine from months 6-12. The aim is to assess the maintenance of weight loss for subjects initially treated with sibutramine + behavior therapy. 5. To compare the two conditions in #4 and #5 above during both Phase 1 and 2: 1) waist circumference; 2) blood pressure; 3) serum lipids; 4) plasma glucose; 5) psychological status; and 6) eating behavior.